1. Technical field
This invention is in the general field of microwave devices and, more particularly, devices having at least an electrically tunable ferroelectric layer and at least a magnetically tunable ferromagnetic layer in close proximity.
2. Background
The microwave region of the spectrum ranges very approximately from 1 to 300 GHz with wavelengths of 30 cm to 1 mm, respectively. Most applications use frequencies of less than 50 GHz. In general terms, most microwave devices comprise two or more conductors on, enclosing, or enclosed by non-conductive media. One characteristic of the media is the propagation constant, xcex2, given by:
xcex2=xcfx89(∈oxcexco∈excexce)xc2xdxe2x80x83xe2x80x83Equation (1)
where xcfx89 is the radian frequency and ∈e and xcexce are the effective dielectric constant and permeability of the media, respectively. (For air, ∈e,=xcexce=1). Microwaves traveling along a transmission line experience a delay, usually expressed as a propagation phase difference given by:
xcfx86=xcex2l=xcfx89l(∈oxcexco∈excexce)xc2xd(radians)=360(l/xcex)(∈excexce)xc2xd(degrees)xe2x80x83xe2x80x83Equation (2)
where l is the effective distance between the two points and xcex is the free-space wavelength. Thus, changing or tuning any of l, ∈e or xcexce changes the phase difference. However, in principle, any delay line configuration can also be viewed as a frequency filter structure providing low insertion loss in pass bands and high attenuation in stop bands. Fundamentally, a phase shifter or filter can be viewed as the same device with emphasis on different physical quantities (governed by the propagation constant xcex2). The most useful tunable microwave devices are phase shifters, resonators, and frequency filters.
Conventional microwave tuning is predominantly achieved via changing the physical is dimension with tuning screws, tuning plungers, sliding conductors or sliding walls. Typically, the transmission line is a hollow rectangular waveguide. Mechanical tuning, however, has the major drawbacks of bulkiness, inconvenient operation, and low tuning speed.
Microwave transmission lines can be made on circuit boards. G-10 epoxy (∈e=10, xcexce=1) is useful to 1.5 GHz, where it becomes too lossy, and Teflon(copyright) (∈e=2.3, xcexce=1) at higher frequencies. Typically, the transmission line consists of a ground plane on one side and strip conductors on the other. However, if the dielectric constant is high enough, coplanar conductors on one side only, can be used. This is because the electric fields will be concentrated in the substrate. A mixture of Teflon(copyright) and ceramic powders produce an ∈e=10. Devices are constructed with components including active monolithic microwave integrated circuits (MMICs) connected by such transmission lines. Tuneable oscillators can be constructed with yttrium iron garnet (YIG) spheres or barium titanate (BTO) cylinders placed in close proximity to the transmission line. These produce a resonance in a circuit whose center frequency can be shifted by application of an external magnetic or electric field, respectively. See Handbook of Microwave and Optical Components, Vol. 1, K. Chang, ed., Wiley Interscience (1997), incorporated herein by reference, for background material on traditional microwave devices.
Microwaves are used for both communications and radar. In both, but especially for radar, the beam direction can be steered by using a two dimensional array of phase shifters. For some applications, several thousand are required and there is an incentive to make the shifters as compact as possible. One solution, ca. 1980, uses YIG as the medium in contact with a coplanar transmission line. The permeability, xcexc, of YIG (∈e=15, xcexce=on the order of 1,000) can be varied by the application of an external magnetic field, H, because xcexce is non-linear with H. Therefore, the phase shift of the RF wave can be varied according to Equation 2. Single crystals of YIG are available, but only in small sizes at great cost. However, polycrystalline YIG can be used with low loses even at high power levels. Also, its properties can be varied by changing the chemical composition. It is the preferred technologically important material for many microwave applications.
In the last decade, similar microwave devices based on varying the permittivity, ∈, of a ferroelectric substrate using a voltage between the transmission line conductors have been proposed. The technological material of choice appears to be BaxSr1xe2x88x92x TiO3 (BST) (x=0 to 1) (∈e=on the order of 1000, xcexce=1). The advantages of using BST for tunable microwave applications are its low loss and high tunability, i.e., variation of permittivity with voltage. Tunability is maximized when the material is operated near its Curie temperature. For BST, this can be adjusted by changing the Ba concentration, x. For example, at least for bulk BST, it ranges from 30 K to 400 K for Ba concentrations ranging from x=0 to 1, respectively. The ability to control the dielectric properties of BST in a simple way allows device structures to be easily optimized for maximum tunability and minimum loss at the desired frequency and operating temperature. In addition, for rapid tuning, it is generally easier to provide rapidly changing electric fields than magnetic ones.
Individually, each approach, ferromagnetic or ferroelectric, has limitations. In a phase shifter, ferromagnetic tuning is limited to frequencies less than the ferromagnetic resonance frequency. Ferroelectric tuning is limited by voltage breakdown. For a phase shifter, arbitrarily large phase changes can be obtained by making the transmission line arbitrarily long, but large sizes are generally not desirable. A structure that combined both approaches would have the advantage of producing additional phase shift for the same length. A non phase-shifter example is a multi-element filter where the center frequency, passband, and ripple are determined in a complicated but well known way by conductor geometries and the propagation constant, xcex2. The filter characteristics can be changed by changing xcex2. Providing a separate magnetic field for each element would be inconvenient, but a purely electrical device may not provide enough change in xcex2. Using dual tuning, the magnetic field could provide the broadband tuning of the filter bandpass while electric tuning for each element would allow fine tuning of the filter profile to achieve symmetric and optimum filter characteristics.
One of the problems with electrical or magnetically tunable devices is that the transmission line impedance also changes. Even though initially connected to a matching impedance, changes will introduce unwanted reflections. The impedance is given by:
Zxe2x88x9d(xcexce/∈e)xc2xdxe2x80x83xe2x80x83Equation (3)
with the proportionality being determined by geometrical factors. For most devices, it would be a large advantage to maintain a constant ratio between xcexce and ∈e and therefore a constant Z.
One application where a dual tuning capability would be most advantageous is for phased-array antennas, see U.S. Pat. Nos. 5,309,166 and 5,589,845, incorporated herein by reference, for ferroelectric only versions. Magnetic fields that are difficult to apply to individual elements could be used for overall steering and electric fields for fine tuning each element. In this application, the slower magnetic tuning could be used for gross adjustment and the faster electric tuning for fine adjustment.
Using a material that has the properties of both might seem obvious. Such materials, e.g., europium barium titanate, have been known to exist for a long time. They are, however, not very sensitive to either electric or magnetic fields. Ferroelectric-ferromagnetic composites are also known and have been used to filter out high frequency electromagnetic interference in feed through connectors. The composites are made by sintering fine grain (0.1 um to 1.0 um) oxide powders. However, it is believed that the change in permitivity of the ferroelectric material with an applied electric field would be very low. Thus, no single material having the properties of both has been used for microwave devices. Moreover, no composite material microwave substrate has been produced combining both voltage and magnetic tuning capabilities.
Accordingly the main object of the invention is to provide a dual tunable microwave substrate comprising practical ferroelectric and ferromagnetic material so that those skilled in the art can use the substrate in various geometrical configurations to build phase shifters, delay lines, resonators, oscillators, directional couplers, filters, various antennas, and other microwave and millimeter wave devices that can be controlled or tuned by application of a voltage and/or magnetic field.
A further object of the invention is to provide a dual tunable microwave substrate whose propagation constant can be controlled while maintaining a relatively constant transmission line impedance.
These objectives are realized by depositing a ferroelectric thin film on a ferromagnetic substrate. High quality ferroelectric films are produced by using one or more intermediate layers as buffers for epitaxial growth. One example uses a yttrium iron garnet substrate with a buffer layer of yttrium stabilized zirconia and barium strontium titanate as the ferroelectric thin film. Another example uses buffer layers comprising silicon nitride followed by magnesium oxide. Pulsed laser deposition or a metal-organic chemical liquid deposition process which is novel with respect to the ferroelectric films can be used to deposit them.
An example of a microwave device is a phase shifter constructed by depositing coplanar gold electrodes on the ferroelectric thin film. A two dimensional array of these can be used to construct a phased array antenna. Other examples are frequency filters and resonators. Performance of all devices can be improved by using high temperature superconductors as coplanar electrodes. A microminiature magnetic field generator is useful in some applications.